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When you go to the dentist to have your jaws X-rayed, you’ll notice that you have to wear a heavy lead apron—and the person who takes the X-ray leaves the room to do it. X-rays are safe, but these precautions give you some sense of how careful we have to be around even relatively harmless levels of radiation. The problem is that unsafe levels of radiation can mess with the cells and molecules in our body, warping them and even breaking them apart. It does the same thing to microbial cells.

But there’s one bacterium called Deinococcus radiodurans <die-no-cah-cuss radio-dur-anz> that can live through blasts of radiation thousands of times greater than the level that would kill a human being. Radiation is measured in units called rads. A dose of 500 to 1,000 rads is enough to kill a person. D. radiodurans thrives even after being hit by up to 1,500,000 rads—yikes!

The bacterium’s Latin name means "strange berry that withstands radiation." Scientists who study D. radiodurans have nicknamed it "Conan the Bacterium." The Guinness Book of World Records dubbed it the world’s toughest bacterium.

So how does D. radiodurans manage to live through such intense blasts of radiation?

If you’ve read other pages on this site, you may have learned that some bacteria form protective spores to survive through drought, heat and radiation (see How Microbes Form Spores). So could it be that D. radiodurans forms extra tough spores?

Actually no, it’s not a spore-former. Anyway, while bacterial spores can endure radiation, none can take nearly as much as D. radiodurans can. So, without any special protective coating, what then does D. radiodurans do to withstand all that radiation?

Let’s first take a quick look at what radiation does to a living thing. When a creature gets hit by a high dose of radiation, the intense energy causes the large DNA molecule in each cell—the collection of all the genes that make a living thing what it is—to fall apart. No creature can survive without its genes in working order.

Most microbes have tools they use to repair occasional damage to their DNA. For example, when a break happens in the DNA of an E. coli bacterium (one of the common bacteria living in your gut), the bug can usually repair it and get on with daily life. However,E. coli usually can’t survive more than two or three major breaks in its DNA. D. radiodurans, on the other hand, can stitch back together DNA shattered to bits by radiation within a matter of a few hours.

One reason is that it has lots of extra copies of its genes. D. radiodurans cells have four to ten copies of their DNA molecule. Most bacteria have only one copy. These copies serve as back-ups, kind of like the back-up copies you make of your important computer files in case one day your computer crashes. So when radiation hits and D. radioduran’s DNA gets busted up, the microbe has lots more chances of finding an intact copy of each gene to use as it stitches its DNA back together. A special protein called RecA does the stitching. Also, it appears that D. radiodurans may have more of the cell repair tools that most bugs have.

Still, scientists aren’t entirely clear exactly how and why D. radiodurans is so much more radiation-resistant than other microbes. After all, other microbes have many of the same tools, although not in as high numbers and variety as D. radiodurans does. And some other bacteria have more than one copy of their genes, although none have quite as many copies as D. radiodurans. Scientists are sifting through the genes of D. radiodurans, trying to figure out if the bacterium has any unique tools or genes that give it extra radiation protection.

They’re also trying to figure out why D. radiodurans evolved this super radiation-resistance in the first place. After all, there aren’t any places on the planet where the microbe would have been exposed naturally to such incredibly high radiation blasts. It turns out that D. radiodurans is also amazingly able to live through long periods with absolutely no water without turning into a tiny dried-out husk. Some researchers think that the bacterium’s radiation resistance is a lucky side effect of the ability the bug evolved to withstand long periods without water—a common natural occurrence. This is because dehydration causes the same kinds of breaks in DNA as radiation does and requires the same patching process to fix these breaks.

However it does what it does, D. radiodurans is indeed the toughest of the tough in the microbial world.